This invention relates to a circuit arrangement for deriving a direct voltage from an alternating voltage, particularly the mains alternating voltage, said circuit arrangement comprising:
two input terminals, PA1 two output terminals one of which is connected to one of the input terminals, PA1 a switching transistor whose main current path is coupled between the other input terminal and the other output terminal, PA1 a base control circuit for controlling the switching transistor, which base control circuit includes a thyristor which is coupled via a resistor between the two input terminals, the node between the resistor and thyristor being coupled to the control electrode of the switching transistor, and PA1 a time-determining circuit comprising a series arrangement of a resistor and a capacitor and a trigger element connected between the gate terminal of the thyristor and the node between the said resistor and capacitor.
A circuit arrangement of this type is known from British Patent Specification GB No. 1,252,637. In this known circuit arrangement the series arrangement of resistor and capacitor, which forms part of the time-determining circuit, is directly connected between the input terminals and the trigger element is constituted by a neon lamp. A fullwave rectified alternating voltage must be presented to the input terminals. At the start of each half cycle the capacitor in the RC series circuit will be charged until the ignition voltage of the neon lamp is reached. Ignition of the neon lamp will render the thyristor conducting so that in its turn the switching transistor is turned off.
The operation of this known time-determining circuit is very inaccurate. On the one hand the neon lamp is an element having ample tolerances. On the other hand the residual voltage across the capacitor at the start of a subsequent cycle is not always the same, which together with the tolerances of the resistor and the capacitor leads to ample margins in the instant when the neon lamp and hence the thyristor are ignited. Consequently, considerable variations in the amplitude of the direct voltage may occur at the output terminals, which particularly applies when relatively low direct voltages must be obtained from relatively high alternating voltages. This is undesirable or inadmissible for many applications, for example, for the power supply of low-voltage elements such as small lamps, light-emitting diodes and the like.
A circuit arrangement of the type described in the opening paragraph can be used, for example, for supplying a control circuit and/or indicator lamps. The circuit arrangement according to the invention is particularly suitable for use in a domestic appliance, like a vacuum cleaner.